The study emphasizes the importance of adding the right number of connections to maintain network operations. Adding too many connections increases costs and burdens resources, while too few lead to fragmented networks that fail to meet user demands. This balance could pave the way for designing quantum networks optimized for rapid computing and secure communication.
"Many researchers are putting significant efforts into building larger and better quantum communication networks around the globe," said Northwestern's Istvan Kovacs, the study's senior author. "But, as soon as a quantum network is opened up to users, it burns down. It's like crossing a bridge and then burning it down behind you. Without intervention, the network quickly dismantles. To tackle this problem, we developed a simple model of users. After each communication event, we added a fixed number of bridges, or links, between disconnected nodes. By adding a large enough number of links after each communication event, we maintained network connectivity."
Kovacs, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences, specializes in complex systems.
"Quantum entanglement is the spooky, space-time-defying correlation between quantum particles," Meng said. "It's a resource that allows quantum particles to talk to each other, so they can perform complex tasks together while ensuring no eavesdropper can intercept their messages."
However, when entangled links are used for communication, they become unusable afterward, fundamentally altering the network. "In classical communications, the infrastructure has enough capacity to handle many, many messages," Kovacs explained. "In a quantum network, each link can only send a single piece of information. Then it falls apart."
To counteract this, the team explored how many new links were needed to maintain the network. They discovered that the critical number of links to add equals the square root of the total users. For instance, in a network with 1 million users, 1,000 links need to be re-added for each qubit of information sent.
"It would be natural to expect that this number increases linearly with the number of users, or maybe even quadratically, as the number of user pairs that could communicate," Kovacs said. "We found the critical number actually is a very small fraction compared to the number of users. But, if you add fewer than that, the network will fall apart, and people cannot communicate."
This insight could guide the design of robust quantum networks capable of recovering from failures by automatically adding new links when others disappear. "The classical internet was not built to be fully robust," Kovacs noted. "It naturally emerged due to technological constraints and user behavior. It was not designed, it just happened. But now we can do better with the quantum internet. We can design it to ensure it reaches its full potential."
Research Report:Path percolation in quantum communication networks
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